The onboard navigation aid systems in aircraft these days routinely or obligatorily include ground collision risk warning systems, such as the TAWS system (“Terrain Awareness and Warning System”), which make it possible to drastically reduce the accident rate. Various TAWS type products are available on the market such as the EGPWS system (Enhanced Ground Proximity Warning System) marketed by Honeywell, or even the T2CAS system marketed by THALES in collaboration with L-3 communications. As a general rule, the main function of these systems is to signal the need to modify the path of the aircraft in the more or less short term, to avoid a collision with the relief or obstacles on the ground, or entry into an overflight-prohibition zone. For these purposes, they use an algorithm for predicting collisions with the ground according to a “terrain” or geographic environment of the aircraft that they must determine, and a prediction that they make concerning the path of the aircraft. The determination of the terrain environment is obtained by correlating the position of the aircraft calculated on the basis of the navigation parameters supplied by the onboard flight management system, notably heading, altitude, latitude, longitude, with a terrain elevation database. The prediction of the path of the aircraft is based on navigation parameters, in particular heading, altitude, latitude, longitude, vertical and ground speed, weight of the aircraft and flight profile models for the aircraft concerned. These monitoring systems supply in particular as output: graphic information to a navigation screen, making it possible for example to display in plan view a map of THD (“Terrain Hazard Display”) type with relief zones by hazard levels indicated in false colours (green, red, etc.) and forecasts of possible conflicts with this relief; audio information to the audio system of the aircraft, to generate if necessary an audible alarm, typically an alarm message. FIG. 1 diagrammatically illustrates such a system. It uses information from a terrain elevation topographic database DB1, onboard or accessible onboard, and a database DB2 of vertical flight profiles of the aircraft concerned, and the various flight parameters P transmitted by the flight management system which will include (the list is not exhaustive): vertical and ground speeds, flight angles, latitude, longitude, altitude, radio-altitude, weight of the airplane and so on. The system mainly comprises three basic functions which feed input data to a collision prediction algorithm 1, which supplies as output in particular graphic display data representing THD (“Terrain Hazard Display”) zones intended to be presented on a navigation screen ND of the cockpit or HSI (“Horizontal Situation Indicator”), control data to an associated control panel CP, and alarm data to the aircraft audio system AAS. These three basic functions are: a function 2 for determining the current position of the aircraft; a function 3 for predicting the flight profile in the near future, based on the current position determined by the function 2, and information from the database DB2; a function 4 for determining the relief of the operating zone of the aircraft, based on the current position determined by the function 2, and information from the database DB1.
These systems are well known to those skilled in the art and have demonstrated the benefit of their use in collision prevention.
In the invention, interest is more particularly focused on the graphic display of the cartographic data supplied by these systems on an onboard screen. This display is currently provided in a form identical to that obtained with a radar. The display devices used initially devolved in effect from meteorological radar systems, such as the WXR system, the commercial name of the product marketed by Rockwell Collins. These systems make it possible to determine meteorological conditions by means of a meteorological radar onboard the aircraft, and display map data of meteorological type. In this case, the navigation parameters are correlated with captured data and processed in real time by the meteorological radar.
The graphic display is thus provided in the form of radials. A radial corresponds to a direction of acquisition by the radar. It is represented by a line, whose origin represents the position of the aircraft and whose direction corresponds to the bearing angle between the directions of the aircraft and of the acquisition by the radar, by an aperture angle which defines the resolution of the information and by successive points on this line, displayed in false colours, which represent the values measured by the radar.
Although they do not use active sensors such as the meteorological radar, the TAWS systems, such as in particular the abovementioned EGPWS or T2CAS, use this method of graphic display by radials: they calculate the points of the radials by scanning, in the manner of a radar, topographic data which is a digital representation of the terrain being flown over, extracted from or contained in a terrain elevation database which can be onboard, or downloaded by radio transmission as and when required according to the zones being flown over. Depending on the available display modes (rose, arc, etc.), false colours are used corresponding to relief altitude measurements, absolute or relative to the altitude of the aircraft. This display mode is standardized in official technical recommendations. For the TAWS systems, these technical recommendations are, for example, described in the certification document TSO-c151b, TSO being an acronym for “Technical Standing Orders”. The mapping data obtained from applicable calculations implemented in these systems is structured in formatted frames compliant with the ARINC 453 protocol to be delivered to a graphic display management system.
This graphic display mode based on radials does, however, have some drawbacks, including:                the time to update a complete image, corresponding to a 360° sweep of the operating zone of the aircraft, is slow; if the position of the aircraft is superimposed on the map background, the latter is offset relative to the position of the aircraft, an offset which is marked if the aircraft is accelerating or turning.        the display graphic data is not geographically referenced (or geo-referenced), for example with a latitude/longitude and an orientation relative to geographic north, because of the very nature of the radar-type sweep. It is only fixed relative to the origin of the radials, or the instantaneous position and orientation of the aircraft. It is thus difficult to superimpose on the map background image other graphic information, such as the flight plan. Now, research efforts are geared towards combining a variety of graphic information on the same navigation screen, to facilitate understanding by the pilots.        the position at the origin of each radial is no longer geo-referenced. For these reasons, it is not possible to follow in real time the path of the aircraft: it is not possible to have the radials rotated or shifted with the aircraft. The next update of the radials must be awaited. The effect induced is a “fixedness” of the screen background displaying the map (the relief), while other graphic elements follow the movement of the aircraft, in particular the flight plan. There is therefore a display inconsistency with a delay effect. This effect can be mitigated by increasing the update frequency, but this incurs a cost overhead in time and computation resources.        the minimum resolution of the system that supplies the map data to be displayed is defined relative to the maximum range of the radials, which, for a given range, and a given application system, defines the angular aperture of the radials and the size of the points on each radial. Thus, in a given operational situation, the size of the points along the radials and the angular aperture of the radials are constant. FIG. 2a represents a radial R. The angular aperture α of the radial defines for each point pi a small zone roughly similar to a trapezoid shape t (the edges of the “trapezium” t here being arcs of circles), on which a hazard level is calculated, typically the highest altitude on said trapezium. The angular aperture α also corresponds to the angular difference between two successive radials, that is, it defines the angular sampling of a segment. FIG. 2b illustrates a set of radials of origin O with a constant angular aperture α, which digitizes a part of the space surrounding the aircraft. If this angular aperture is large, a point pi near the origin O will be calculated several times, for example it will be calculated for the radials R5, R6, R7 . . . , as illustrated in FIG. 2b which for example represents a forward segment. At the extreme, the point at the origin O is calculated and displayed for each radial. This implies an overhead in time and computation resources to calculate the radials and display them.        a data conversion is needed to switch from the “radial” type map information format, with points as polar coordinates relative to a point of origin, to a “map” type display format in a discretized space with pixels as Cartesian coordinates. This conventionally introduces graphic artifacts, in particular moiré effects.        
Thus, there is a need to enhance the display of the so-called map data, to make it possible to better follow the movement in real time of the aircraft, with an enhanced visual rendition, and to make it possible to display on one and the same screen graphic information obtained from different applications, coherently, while optimizing the necessary computation times and resources.
In the state of the art, all of the 360° convolution zone around the aircraft is calculated. Typically, in a TAWS system, almost 400 radials are calculated corresponding to an image, requiring 100 real time cycles of the associated computer for a complete refresh. In practice, this represents two to four seconds.